The problem of overweight and obesity in active duty military personnel in the U.S. parallels its civilian population, reaching an all-time high of 61% or overweight and 12% for obesity. The high prevalence of overweight and obesity and inadequate fitness in active duty personnel not only impacts overall wellbeing of service members but also have significant negative consequences for a) military readiness and b) the costs associated with early attrition, reduced productivity, and medical care. The Navy and Marine Corps Public Health Center and the Navy Bureau of Medicine and Surgery are at the forefront of addressing weight and physical fitness in military personnel by developing and implementing the ShipShape (SS) program: the Navy's official program for weight management. Although SS is an evidence-based program, on average only 45% of those who complete the program are able to pass the Navy body composition assessment (BCA). The major shortcoming of SS is that it does not address the underlying psychological and emotional issues that may influence pathological eating behavior and poor exercise habits. An innovative and promising way of improving weight and fitness outcomes is to incorporate components of acceptance and commitment therapy (ACT) in order to (a) bolster participants' commitment to behavior change, (b) build distress tolerance skills, and (c) promote values-based weight-related behaviors and goals. These psychological flexibility skills should work synergistically with the dietary and physical activity recommendations of SS to improve the BCA pass rate. The aims of the proposed cohort-randomized controlled trial are to: 1) determine the effectiveness of ACT- enhanced SS (ACT+SS) compared to the standard SS-only program in 280 active duty Navy personnel; 2) examine psychological flexibility as a mechanism underlying intervention response; and 3) explore potential moderators of intervention response. Effectiveness will be measured by assessing the 6-month official BCA pass/fail status, as well as changes in body fat %, physical activity, problem eating, and quality of life from baseline to mid- and post-intervention, and 3- and 6-month follow-up. Other assessments include measures of emotional distress and psychological flexibility. Several unique factors including collaboration with the Naval Center for Combat and Operational Stress Control, active support from the Director of the Health and Wellness Department at Naval Medical Center San Diego, access to the organizational structure of the SS program for recruitment, and an internationally recognized scientific team with expertise in ACT, weight management, physical activity, and military health research ensure the successful completion of this project. Given the significant costs of overweight, obesity, and inadequate physical fitness in active duty personnel, an innovative intervention that significantly enhances weight-related outcomes in a real-world setting could be a ground- breaking step for health and wellness in U.S. service members and the military at large.